Insulative ceramic substrates such as an aluminum nitride substrate and a silicon nitride substrate are used as mounting boards for various kinds of semiconductor elements represented by optical semiconductor elements such as a laser diode and a photodiode. When a ceramic substrate is applied to a sub-mounting board for an optical semiconductor element, a wiring layer is formed on a surface of the ceramic substrate by using a thin film forming technique such as a PVD method such as a vacuum deposition method or a sputtering method, or a CVD method (see, for example, Patent Reference 1).
FIG. 4 is a cross-sectional view showing the structure of a conventional ceramic wiring board. In this drawing, 1 denotes an insulative ceramic substrate made of, for example, an aluminum nitride sintered compact, and on its surface, a main conductor layer 4 made of Au is formed via a base metal layer 2 made of Ti and a first diffusion preventive layer 3 made of Pt. On the main conductor layer 4, at a connection part (electrode connection part) with a semiconductor element, a solder layer 6 made of an Au—Sn alloy is formed via a second diffusion preventive layer 5 made of Pt or the like. A surface of the solder layer 6 is sometimes covered by an Au layer 7 for oxidation prevention.
In the ceramic wiring board shown in FIG. 4, a conductor layer in which a base metal layer 2, a first diffusion preventive layer 3, and an Au layer (main conductor layer) 4 are stacked in this order is formed also on a lower surface side of the insulative ceramic substrate 1. The conductor layer on the lower surface side is used as a bonding metal layer when the insulative ceramic substrate 1 is mounted on an external circuit board or in a package. The conductor layer on the lower surface side is sometimes used as a grounding conductor layer or the like.
The aforesaid second diffusion preventive layer 5 interposed between the main conductor layer 4 and the solder layer 6 prevents Au of the main conductor layer 4 from diffusing into the solder layer 6 made of the Au—Sn alloy or the like when the semiconductor element is bonded and fixed via the Au—Sn alloy or the like of the solder layer 6. If Au of the main conductor layer 4 diffuses into the Au—Sn solder alloy, the alloy comes to have an Au-rich composition and increases in melting point, so that the Au—Sn alloy cannot be melted completely at a soldering temperature (heating temperature). This results in a decrease in bonding strength and the like.
However, in the conventional ceramic wiring board, when the wiring board is heated in order to bond the semiconductor element, the bonding strength is sometimes decreased due to poor wettability between Sn in the solder layer 6 and Pt forming the second diffusion preventive layer 5. It has been further found out that Sn in the solder layer 6, when heated, diffuses into the second diffusion preventive layer 5, which causes the formation of voids near an interface between the solder layer 6 and the second diffusion preventive layer 5. The formation of such voids, if any, makes it difficult to firmly bond the wiring board and the semiconductor element. In addition, the voids increase electrical resistance of the connection part, which may possibly increase an operating current of the semiconductor element.
In particular, to mound an optical semiconductor element such as a laser diode on the wiring board, an Au—Sn alloy is mainly used as described above. Since the Au—Sn alloy is hard and fragile, a thermal load or the like at the time of bonding may possibly cause characteristic deterioration of the semiconductor element. To avoid such problems, the use of a Sn alloy such as a Sn—Cu alloy or a Sn—Ag alloy softer than the Au—Sn alloy has been considered. However, generally having a higher Sn concentration than the Au—Sn alloy, these Sn alloys easily react with the diffusion preventive layer (Sn diffuses into the diffusion preventive layer), resulting in easy formation of voids near the interface.
Patent Reference 1: JP-A 2002-252316 (KOKAI)